The present invention relates to a process for forming a vitreous layer on a refractory surface, in which a vitrifying agent is projected by means of an apparatus against the said surface with an oxygen-containing carrier gas and simultaneously with a combustible gas, the latter generating a combustion flame.
High-temperature furnaces used for various industrial applications may be subjected to a certain amount of degradation over time. It is found that dust or by-products coming from the raw materials and/or from their reaction products accumulate on the various refractory surfaces of the furnace. This-phenomenon is particularly important in coke ovens where the combustion of coal generates the formation of graphite carbon on the refractory surfaces and into the thickness of the refractories, where it may cause cracking. This carbon builds up particularly in the region of the feeding ports, the roof, the flue of the oven and the space between the door and the jambs. Not only does this build-up embrittle the refractory materials but it also decreases the level of charging of the oven. In addition, the cracking is a source of pollution. It is also observed that the mechanical extraction of the coke is made more difficult because of the friction existing between the coke and the graphite carbon deposited on the refractory surfaces. The build-up of graphite carbon in the charging ports also slows down the rate of charging of the oven.
Currently, in order to remove the graphite carbon it is necessary to stop the oven and burn off the graphite carbon to generate CO2. This process, given its slowness, results in the loss of productivity and, moreover, can create local overheating in the refractory bricks, something which may in course of time damage the oven. In the feeding ports, mechanical cleaning is sometimes necessary in order to remove the graphite carbon, which most particularly damages them.
Patent Application EP 908 428 A1 (Kawasaki Steel Corporation) proposes the application in the carbonizing chamber of the oven of a vitrifying agent containing predominantly silica and/or Na2O and to prevent the graphite carbon from adhering to the exposed surfaces. The process involves spraying an aqueous solution or a suspension in water of this agent onto a surface while maintaining the temperature of the latter at 900° C. or more for at least 30 minutes.
Patent Application EP 773 203 A1 (Asahi Chemical Company) describes a similar process for forming a layer of metal oxides on the walls of a coke oven. The process consists in the hot application, using conventional methods, of an aquceous solution or a suspension in water of metal oxide precursors.
In both these methods, the water which comes into contact with the hot refractory surface causes a thermal shock which embrittles the refractory bricks. Silica, of which the refractory bricks are composed, contains a small amount of lime (CaO) which, in the presence of water, is converted into hydrated lime (Ca(OH)2) This hydration causes these bricks to crumble.
The vitreous layers formed according to these processes are generally very thin and tend to wear away rapidly.
Patent Application JP 58-33189 (Kurosaki & Nippon Steel) describes the formation of a vitreous coating for repairing the walls of coke ovens by flame-spraying a mixture of vitrifiable oxides. The layer thus formed tends to crystallize over time, which causes it to crack. To remedy this drawback, Patent Application DE 38 03 047 A1 (Kurosaki & Nippon Steel) describes the formation of a vitreous coating having a high silica content which contains, during its formation, at least 60% of a crystalline phase.